As I'm sure you know, the World-Wide Web (WWW) is the collection of documents accessible on the Internet which reference one another by their Internet-based addresses (URLs and URIs), thus forming a tangled web of related information. The documents are obtained chiefly through the HTTP protocol, and are written in HTML (or its relatives XML and XHTML), plus a load of other data formats and programming languages used to augment it (Java, JavaScript, PNG, JPEG, SVG, etc).

It is HTML (Hypertext Mark-up Language) which makes the web what it is, so you might think that HTML's primary goal – the ability to express hypertext, text containing transparent links to other documents – is its most important feature. That's what makes a collection of HTML documents into the Web, after all. But it was recognised that such a web would become partitioned if some pages were written only for rendering in one size of window or screen, while another set of pages were only for another size. Links from one side of the partition to the other would become useless, as no-one would want to have to reconfigure their browsers each time they traversed a partition – best not to have any reason to partition.

So, a secondary goal (after hypertext) was required to make the primary worthwhile: avoid formatting for a particular medium; don't specify it in the document itself, but allow it to be decided only at the last moment when the document is rendered. While this is only a secondary goal, it is at least as important to attain as the first, which depends on it.

I assert that the best feature (most worthy of exploitation) of the Web is medium independence, followed quickly by hypertext expression. You can see that medium independence can be more important to attain than hypertext because a medium-independent page of text can still be useful even if it has very few links. Medium independence is always necessary, while hypertext is merely (highly) convenient.

The modern Web has many ways of defeating medium independence, such as absolute positioning, fixed widths in tables and CSS, the abuse of <table> for page layout, and unnecessary images that increase the minimum width or are used as the only way to convey some information (useless in non-visual media). If you employ these in your website design, your failure to exploit medium independence utterly defeats the primary aim of the Web.